I have used hypertufa directly over styrofoam with great success. My only caution would be to keep the mixture from becoming too loose as that will cause excessive sagging. Also, you may need to hold each "handful" of your mixture for several seconds until it adheres on its own. I also come back after an hour or so to hand pat areas that have moved a bit on vertical surfaces. Cutting grooves into the vertical surfaces using a cerrated knife is also helpful. Good luck!

If you want your hypertufa to be extra strong, wrap your styrofoam with a layer chicken wire - that will also give the tufa something to hold on to.

I am an infrequent poster here but have tried using concrete and hypertufa over styrofoam boxes that are used to ship pharmacetucicals. They look like a small cooler. I would encourage you to cover the inside completely because the roots of the plants will invade naked styrofoam and eventually destroy it. This is a lesson I learned the hard way.
spiderwoman